Recently, as for angular velocity sensors of relatively small sizes, many efforts have been directed to the development of vibration type angular velocity sensors. The angular velocity sensors of this type are generally arranged to detect the angular velocity electronically from a displacement of a vibratory member under the influence of Coriolis force which is known to be proportional to the angular velocity, through the use of piezoelectric elements located in association with the vibratory member.
The angular velocity sensors of this nature usually involve a fabrication process of extremely high precision, which is very expensive, for example, a fabrication process utilizing semiconductor fabrication techniques (as proposed in Japanese Patent Laid-open No. Sho61-114123 and Sho61-139719).
In this connection, FIG. 12 illustrates a prior art angular velocity sensor as disclosed in the above-mentioned Japanese Patent Laid-open No. Sho61-139719.
In FIG. 12, indicated at 1 is a silicon substrate structure largely constituted by a lower substrate 1A which is shown on the lower side in that figure, an upper substrate 1B which is located over the lower substrate 1A, and a space 1C which is formed between the two substrates 1A and 1B to permit vibration of a cantilever type support beam 2 as will be described hereinlater.
Denoted at 2 is a thin-walled cantilever support beam which is formed integrally in the upper substrate 1B of the substrate structure 1 by etching technology or the like. The cantilever support beam 2 is fixed at one end, that is, at its base end 2A which is fixedly connected to the upper substrate 1B, and disposed freely at the other end 2B for vibration in upward and downward directions perpendicularly to the face of the substrate 1. Further, a rectangular slot 3 is formed longitudinally and centrally of the cantilever support beam 2 in a base end portion 2A thereof.
The reference 4 denotes an electrode which is formed on the top surface of a fore free end portion 2B of the cantilever support beam 2. The electrode 4 is connected through a lead wire to an oscillator circuit which generates a predetermined frequency signal (both of the lead wire and the oscillator circuit not shown). Upon applying the predetermined frequency signal to the cantilever support beam 2 through the electrode 4, the cantilever support beam 2 is vibrated up and down by the electrostatic force which occurs between the cantilever support beam 2 and the lower substrate 1A.
Designated at 5 are piezoresistance elements which are located on the opposite sides of the slot 3 in a base end portion 2A of the cantilever support beam 2 to detect, by way of a variation in resistance, the degree of stress to which the cantilever support beam 2 is subjected when the substrate structure 1 is put in rotation. Output signals of the piezoresistance elements 5, indicative of the angular velocity in the rotational direction, are fed to a signal processing circuit (not shown).
In case of the prior art angular velocity sensor of the above-described construction, upon applying a predetermined frequency signal from the oscillator circuit through the electrode 4, electrostatic force is produced to vibrate the fore end portion 2B of the cantilever support beam 2 up and down in the vertical direction, for example, at its own resonance frequency. In this vibrating state, if a torque T about a rotational axis O is applied to the substrate structure 1, the cantilever support beam 2 is subjected to a certain degree of torsional strain (stress) under the influence of Coriolis forces F and F'. This rotational strain exerts compressive stress C and tensile stress E on the opposite sides of the slot 3. As a result, the piezoresistance elements 5 produce signals commensurate with the compressive stress C and the tensile stress E, respectively, which are indicative of the angular velocity of the applied torque T.
In this regard, according to the above-described prior art angular velocity sensor, while vibrating the cantilever support beam 2 up and down, the stresses which correspond to the angular velocity of the torque T are detected from the respective piezoresistance elements 5 on the basis of Coriolis forces. However, the cantilever support beam 2 which is constantly put in vibration in a vertical direction to the substrate structure 1 is susceptible to influences of viscous resistance of air, and therefore involves complication of its vibrational excitation mechanism.
Accordingly, with the above-described prior art angular velocity sensor arrangements, the energy for vibrating the cantilever support beam 2 is wastefully spent by the viscous resistance of air, which results not only in extremely low energy efficiency but also in low sensitivity of the piezoresistance elements 5 in the detection of the angular velocity by limiting the amplitude of their vibrations.
Besides, the prior art angular velocity sensor, which is arranged to detect the torsional strain of the cantilever support beam 2 which is put in vibration, has an inherent drawback that it can afford only low sensitivity for the detection of angular velocity. Namely, according to the prior art which employs one and single cantilever support beam 2 for both vibrational excitation and detection purposes, there always arises a problem that, when the cantilever support beam 2 is put in vibration to utilize Coriolis force, the cantilever support beam 2 is torsioned at its base end 2A by its own vibration. The resulting torsional strains are picked up by the respective piezoresistance elements 5 to increase the background noises to such a degree as to render detection of fine angular velocities difficult or to degrade the angular velocity detection sensitivity and accuracy to an objectionable degree.
In view of the above-described problems or drawbacks of the prior art, the present invention has as its object the provision of an angular velocity sensor which is arranged to suppress the influence of the viscous resistance of gases in association with a vibratory member and to enhace the angular velocity detection sensitivity through reduction of noises.